The present invention relates to a control apparatus for controlling a stepless transmission having a drive pulley and a follower pulley the diameter of each of which is changed when hydraulic pressure is applied or discharged, and, more particularly, to a technology for preventing generation of an excessive engine rotation due to a failure or accident of the transmission.
Hitherto, as a control apparatus for a stepless transmission of the type described above, a structure has been disclosed in Japanese Patent Laid-Open No. 62-4958 and another structure has been disclosed in Japanese Patent Laid-Open No. 63-53356. According to the above-described disclosures, the structure is arranged to comprise a drive pulley (or primary pulley) and a follower pulley (or secondary pulley) the effective radius of each which can be varied, a belt arranged between the two pulleys and a change ratio control valve for applying/discharging hydraulic pressure from the hydraulic cylinder of the drive pulley. As a result, the effective radius of the drive pulley is adjusted by the change ratio control valve and the effective radius of the follower pulley is thereby changed so that the change ratio between the two pulleys is continuously varied.
The change ratio control valve is disposed in the operation oil passage which communicates with the hydraulic cylinder of the drive pulley. Furthermore, a drain passage is connected to the change ratio control valve. Hydraulic pressure is supplied from the operation oil passage to the hydraulic cylinder of the drive pulley. When the hydraulic cylinder of the drive pulley is allowed to communicate with the drain passage, the applied hydraulic pressure is discharged. As a result of the application/discharge of hydraulic pressure, the effective radius of the drive pulley is changed so that the change ratio is controlled.
In a conventional geared transmission, if a certain gear ratio is selected, the selected gear ratio, that is, the change ratio is stably maintained. In the stepless transmission of the type described above, in which the change ratio is controlled by hydraulic pressure, hydraulic pressure must continuously be controlled in order to obtain the desired change ratio. Namely, (1) In order to accurately set the actual change ratio with respect to the target change ratio, hydraulic pressure must be accurately controlled. (2) If the hydraulic control system fails, a means to overcome the failure must be provided. If both of the two conditions (1) and (2) are not met, the engine rotates excessively. The reason why the engine rotates excessively will be described.
In the structure disclosed in Japanese Patent Laid-Open No. 62-4958, when a driver operates an acceleration pedal at a considerably large degree and the engine speed is thereby rapidly raised, in particular, when the change ratio is subjected to a kick-down, the change ratio moves to a considerably larger value. In this case, the change ratio control valve acts to reduce the effective radius of the drive pulley by removing hydraulic pressure by connecting the hydraulic cylinder of the drive pulley to the drain passage. However, even after the change ratio has reached the target change ratio, the change ratio control valve continues to connect the hydraulic cylinder of the drive pulley to the drain passage due to the action of the change ratio control valve performed in a manner such that a certain response delay of the electric control system and the hydraulic system occurs. As a result, the effective radius of the drive pulley is excessively reduced, causing the engine to rotate excessively. The above-described problem will also take place in a case where the change ratio control valve becomes fixed to the side which communicates with the drain passage because of a failure of the magnetic solenoid or sticking.
The structure disclosed in Japanese Patent Laid-Open No. 63-53356 comprises a change ratio control valve for varying the change ratio, a change ratio fixing valve for fixing the change ratio by sealing the hydraulic pressure in the pulley hydraulic chamber, and a control solenoid for switching the previously-described two valves. The stepless transmission disclosed in Japanese Patent Laid-Open No. 63-53356 enables the transmission feeling to be improved by making the change ratio constant by sealing the hydraulic pressure in the hydraulic chamber of the pulley by switching the change ratio fixing valve to the position at which communication is cancelled.
A case in which the above-described change ratio control valve has become sticky will now be considered. In this case, draining of hydraulic pressure in the hydraulic chamber of the primary pulley continues. Therefore, the primary pulley rotates at an excessively high speed. The stepless transmission is arranged in a manner such that the operating fluid pressure level for the primary pulley and the secondary pulley is set so as to obtain the target change ratio, that is, the target ratio between the revolution of the primary pulley and that of the secondary pulley. Therefore, if the revolution of the primary pulley exceeds the target revolution, feedback control is performed so as to prevent the excessive rotation. However, in the case above-described in which the change ratio control valve sticks, the change ratio control valve is not operated if the feedback control is operated. Therefore, abnormal gear change cannot be prevented. That is, in the case where the valve sticks, fail safe control must performed by a system independent of the change ratio control system.
Furthermore, the structure disclosed in Japanese Patent Laid-Open No. 63-53356 comprises the change ratio control valve and the change ratio fixing valve whereby, when the fixing valve is operated, the change ratio can be maintained at a constant ratio regardless of the status of the change ratio control valve. Therefore, it might be considered that the structure is able to have a fail safe control function. However, since the structure disclosed in Japanese Patent Laid-Open No. 63-53356 was developed so as to improve the driving feeling, it has no means to detect a failure and control the change ratio fixing valve when a failure has been detected.